This invention relates in general to an electroplating (electrolytic plating) technique, and, more particularly, to a technique that is effective for application to a plating step in the process of manufacture of a semiconductor device.
In semiconductor devices manufactured by use of lead frames, it is usual to form an alloy skin layer at external connection terminals of a lead so as to secure solder wettability in the course of mounting the semiconductor device on a substrate through soldering. Although an alloy skin layer made of a Pb (lead)-Sn (tin) composition has been predominantly used for such a skin layer, a limitation has been recently placed on the use of Pb from the standpoint of environmental protection. Thus, the use of a Pb-free alloy skin layer has been in development.
Various types of Pb-free compositions have been proposed for use as an alloy skin layer and, in fact, some have been reduced in practice. For example, there a known alloy skin layer made of a Sn—Bi composition (i.e. a Sn—Bi alloy skin layer). This Sn—Bi alloy skin layer is disclosed, for example, in Japanese Patent Publication Hei 7 (1995)-65206. In this publication a technique for forming a Sn—Bi alloy skin layer according to an electroplating method is also disclosed.